The source of the river is a series of springs known as Churn Water Heads, that occur on Bully Hill north of Tealby in the Lincolnshire Wolds.

These springs occur where the permeable chalk geology of the Wolds meets the impermeable mudstone and clays that underlie most of the Rase catchment.

At the hamlet of Bishopbridge the river changes character, becoming an artificial drainage channel that runs in parallel with the Ancholme.

As it flows further north the river also loses its identity becoming the East Drain, when it is joined by the Kingerby Beck.

The waters of the Rase and the Ancholme eventually join together near Harlam Hill, to the south of Snitterby Carr.